Plasma production device and method and RF driver circuit with adjustable duty cycle

ABSTRACT

An RF driver circuit and an orthogonal antenna assembly/configuration, are disclosed as part of a method and system for generating high density plasma. The antenna assembly is an orthogonal antenna system that may be driven by any RF generator/circuitry with suitable impedance matching to present a low impedance. The disclosed RF driver circuit uses switching type amplifier elements and presents a low output impedance. The disclosed low-output impedance RF driver circuits eliminate the need for a matching circuit for interfacing with the inherent impedance variations associated with plasmas. Also disclosed is the choice for capacitance or an inductance value to provide tuning for the RF plasma source. There is also provided a method for rapidly switching the plasma between two or more power levels at a frequencies of about tens of Hz to as high as hundreds of KHz.

REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under U.S.C. §119 and is acontinuation-in-part of Ser. No. 10/268,053 filed Oct. 8, 2000 andclaims benefit of the U.S. Provisional Application No. 60/328,249 filedon Oct. 9, 2001, which is incorporated herein in its entirety byreference.

FIELD OF THE INVENTION

The present invention relates generally to the design and implementationof a plasma generation system. More particularly, it relates to radiofrequency amplifiers, antennas and effective circuit connections forinterfacing the amplifiers and antennas for generating plasma.

BACKGROUND OF THE INVENTION

Plasma is generally considered to be a fourth state of matter, theothers being solid, liquid and gas states. In the plasma state theelementary constituents of a substance are substantially in an ionizedform rendering them useful for many applications due to, inter alia,their enhanced reactivity, energy, and suitability for the formation ofdirected beams.

Plasma generators are routinely used in the manufacture of electroniccomponents, integrated circuits, and medical equipment, and in theoperation of a variety of goods and machines. For example, plasma isextensively used to deposit layers of a desired substance, for instancefollowing a chemical reaction or sputtering from a source, to etchmaterial with high precision, and to sterilize objects by the freeradicals in the plasma or induced by the plasma or to modify surfaceproperties of materials.

Plasma generators based on radio frequency (“RF”) power supplies areoften used in experimental and industrial settings since they provide aready plasma source, and are often portable and easy to relocate. Suchplasma is generated by coupling the RF radiation to a gas, typically atreduced pressure (and density), causing the gas to ionize. In any RFplasma production system, the plasma represents a variable load at theantenna terminals as the process conditions changes. Among other processcontrol factors, changes in working gas and pressure affect the amountof loading seen at the antenna terminals. In addition, the amplitude ofthe RF drive waveform itself affects the plasma temperature and density,which in turn also affects the antenna loading. Thus the antenna/plasmacombination represents a non-constant and nonlinear load for the RFpower source to drive.

A typical RF source has a 50 ohm output impedance, and requires a loadthat presents a matching 50 ohm impedance in order for the RF source tocouple to the load most efficiently. Because of the often unpredictablechanges in the plasma self inductance, effective resistance, and mutualinductance to the antenna, provision for impedance matching is made byretuning some circuit elements and possibly the plasma to obtainsatisfactory energy transfer from the RF source to the generated plasma.To achieve this, an adjustable impedance matching network, or “matchingbox” is typically used to compensate for the variation in load impedancedue to changes in plasma conditions. The matching box typically containstwo independent tunable components, one that adjusts the seriesimpedance and the other that adjusts the shunt impedance. Thesecomponents must be adjusted in tandem with each other in order toachieve the optimum power transfer to the plasma. Not surprisingly,accurate tuning of these components is often a difficult process.Typically, retuning requires manual/mechanical operations/actuators toadjust one or more component values and generally sophisticated feedbackcircuitry for the rather limited degree of automation possible.

It is well known that the application of a sufficiently large electricfield to a gas separates electrons from the positively charged nucleiwithin the gas atoms, thus ionizing the gas and forming the electricallyconductive fluid-like substance known as plasma. Coupling radiofrequency electric and magnetic fields to the gas generates, via anantenna, induces currents within this ionized gas. This, in turn, causesthe gas to further ionize, and thereby increasing its electricalconductivity, which then increases the efficiency with which the antennafields couple to the charged particles within the gas. This leads to anincrease in the induced currents, resulting in the electrical breakdownand substantial ionization of the gas by various mechanisms. Theeffectiveness of the RF coupling is dependent upon the particular RFfields and/or waves that are used. Some types of waves that are suitablefor the efficient production of large volumes of plasma are describednext.

Whistler waves are right-hand-circularly-polarized electromagnetic waves(sometimes referred to as R-waves) that can propagate in an infiniteplasma that is immersed in a static magnetic field B_(o). If these wavesare generated in a finite plasma, such as a cylinder, the existence ofboundary conditions—i.e. the fact that the system is not infinite—causea left-hand-circularly-polarized mode (L-wave) to exist simultaneously,together with an electrostatic contribution to the total wave field.These “bounded Whistler” are known as Helicon waves. See Boswell, R. W.,Plasma Phys. 26, 1147 (1981). Their interesting and useful qualitiesinclude: (1) production and sustenance of a relatively high-densityplasma with an efficiency greater than that of other RF plasmaproduction techniques, (2) plasma densities of up to Np˜10¹⁴ particlesper cubic centimeter in relatively small devices with only a few kW ofRF input power, (3) stable and relatively quiescent plasmas in mostcases, (4) high degree of plasma uniformity, and (5) plasma productionover a wide pressure range, from a fraction of a mTorr to many tens ofmTorr. Significant plasma enhancement associated with helicon modeexcitation is observed at relatively low B_(o)-fields, which are easilyand economically produced using inexpensive components.

Significant plasma density (N_(p)) enhancement and uniformity may beachieved by excitation of a low-field m=+1 helicon R-wave in arelatively compact chamber with B_(o)<150 G. This may be achieved, forinstance, through the use of an antenna whose field pattern resembles,and thus couples to, one or more helicon modes that occupy the samevolume as the antenna field. The appropriate set of combined conditionsinclude the applied magnetic field B_(o), RF frequency (F_(RF)),), thedensity N_(p) itself, and physical dimensions.

Some antenna designs for coupling RF power to a plasma are disclosed byU.S. Pat. Nos. 4,792,732, 6,264,812 and 6,304,036. However, thesedesigns are relatively complex often requiring custom components thatincrease the cost of system acquisition and maintenance. Moreover, notall of the designs are suitable for efficient generation of the heliconmode, which is a preferred mode disclosed herein.

RF power sources typically receive an external RF signal as input orinclude an RF signal generating circuit. In many processingapplications, this RF signal is at a frequency of 13.56 MHz, althoughthis invention is not limited to operation at this frequency. Thissignal is amplified by a power output stage and then coupled via anantenna to a gas/plasma in a plasma generator for the production ofplasma. Amplifiers are conventionally divided into various classes basedon their performance characteristics such as efficiency, linearity,amplification, impedance, and the like, and intended applications. Inpower amplification, an important concern is the amount of power wastedas heat, since heat sinks must be provided to dissipate the heat and, inturn, increase the size of devices using an inefficient amplifier. Aclassification of interest is the output impedance presented by anamplifier since it sets inherent limitations on the power wasted by anamplifier.

Typical RF amplifiers are designed to present a standard outputimpedance of 50 Ohms. Since, the voltage across and current through theoutput terminals of such an amplifier are both non-zero, their productprovides an estimate of the power dissipated by the amplifier. Incontrast to such amplifiers, a switch presents two states: it is eitherON, corresponding to a short circuit, i.e., low impedance, or OFF,corresponding to an open circuit, i.e., infinite (or at least a varylarge) impedance. In switched mode amplifiers, the amplifier elementacts as a switch under the control of the signal to be amplified. Bysuitably shaping the signals, for instance with a matching load network,it is possible to introduce a phase difference between the current andthe voltage such that they are out of phase to minimize the powerdissipation in the switch element. In other words, if the current ishigh, the voltage is low or even zero and vice versa. U.S. Pat. Nos.3,919,656 and 5,187,580 disclose various voltage/current relationshipsfor reducing or even minimizing the power dissipated in a switched modeamplifier.

U.S. Pat. No. 5,747,935 discloses switched mode RF amplifiers andmatching load networks in which the impedance presented at the desiredfrequency is high while harmonics of the fundamental are short circuitedto better stabilize the RF power source in view of plasma impedancevariations. These matching networks add to the complexity for operationwith a switched mode power supply rather than eliminate the dynamicmatching network.

The problems faced in efficient plasma generator design include the needfor a low maintenance and easily configured antenna, the elimination ofexpensive and limited matching networks to couple the RF power source tothe non-linear dynamic impedance presented by a plasma, and the need forRF power sources that can be efficiently modulated.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide animproved antenna design for efficiently coupling RF sources to a plasma.It is yet another object of the present invention to provide systems forgenerating a plasma with the aid of an RF power source without requiringthe use of a matching network to couple the RF power source to theplasma.

An illustrative plasma generator system in accordance with oneembodiment of the present invention comprises at least one plasmasource, the at least one plasma source having an antenna including aplurality of loops, each loop having a loop axis, the plurality of loopsarranged about a common axis such that each loop axis is substantiallyorthogonal to the common axis; at least one radio frequency power sourcefor driving the plurality of loops in quadrature and coupled to a plasmaload driven in a circularly polarized mode, preferably a helicon mode,via the antenna; a static magnetic field substantially along the commonaxis; and a reactance coupling the switching amplifier to the antennaloops such that the reactance and the antenna loops without the plasmahave a resonant frequency that is about equal to a specified frequencyand dispensing with the requirement for a matching network. Thereactance coupling the switching amplifier to the antenna loops ispreferably provided at least in part by a capacitor.

The radio frequency power source preferably comprises at least onemember from the group consisting of a substantially Class A amplifier, asubstantially Class AB amplifier, a substantially Class B amplifier, asubstantially Class C amplifier, a substantially Class D amplifier, asubstantially Class E amplifier, and a substantially Class F amplifier.In one embodiment, these are connected to the primary of a transformerto reduce the drive impedance to a low value. Even more preferably theradio frequency power source includes a Class D amplifier in a push-pullconfiguration with a relatively low output impedance.

In a preferred embodiment, the radio frequency power source exhibits alow output impedance in comparison with the input impedance of anantenna. Often the low output impedance is significantly less than thestandard impedance of 50 Ohm. The output impedance is preferably withina range selected from the set consisting of less than about 0.5 Ohms,less than about 2 Ohms, less than about 3 Ohms, less than about 5 Ohms,less than about 8 Ohms, less than about 10 Ohms, and less than about 20Ohms. Preferably the output impedance is less than 5 Ohms, even morepreferably the output impedance is between 0.5 to 2 Ohms, and mostpreferably the output impedance is less than 1 Ohm. Use of thislow-impedance driver together with the disclosed circuit for connectingthe driver to the current strap of an antenna eliminate the need for amatch box, thus reducing circuit complexity and eliminating a source offailure in plasma processing systems.

A further advantage of the disclosed system is that the voltage appliedto the antenna can be made quite large prior to plasma formation, thusincreasing the ability to initiate the plasma in a variety of workingconditions. Once the plasma is formed, the voltage reduces to a lowerlevel to sustain the plasma, mitigating the harm resulting from possiblehigh antenna voltages.

Depending upon the phasing between antenna elements and the value ofB_(o), the system can be run as a helicon source, or as a magnetizedinductively coupled plasma (MICP) source, or as an ICP source at B₀=0.Furthermore, it is observed to operate efficiently and robustly inpressure regimes (e.g., with P_(o) approximately 100 mTorr) that arevery difficult to access and/or make good use of using prior art plasmasources. The currents in the antenna elements appear to abruptly “lock”into a quadrature excitation mode when the conditions on neutralpressure P₀, input power P_(RF), and externally applied axial magneticfield B_(o), are right. When this occurs, the plasma appears to fill thechamber approximately uniformly, which is advantageous over othersources due to the ability to produce uniform processing conditions.

Additionally, the combination of antenna system plus RF generator cancreate and maintain a plasma under conditions where the plasmaparameters vary over much larger ranges than have been reported forother sources (e.g. neutral pressure Po varied from 100 mTorr down to 5mTorr, and then back up again to 100+ mTorr, in a cycle lastingapproximately one minute), without the need for the adjustment of anymatching network components.

Another advantage of the disclosed system is that the elimination of thematching network can result in an “instant-on” type of operation for theplasma source. This characteristic can be used to provide an additionalcontrol for the process being used. In particular, it is possible tomodulate the amplitude of the RF power that is generating the plasma,between two (or more) levels such as 30% and 100%, or in a fully on-offmanner (0% to 100%). This modulation can occur rapidly, e.g. at afrequency of several kilohertz, and can accomplish several purposes. Forinstance, the average RF power can be reduced with a consequentreduction in average plasma density. The “instant-on” operation cangenerate plasma with an average RF input power of as little as 5 W to avolume of 50 liters.

In addition, modulation can be used to control the spatial distributionof the working gas within the reaction chamber: The distribution of theworking gas is modified by the plasma, which often contributes to thenon-uniformity of fluxes of the active chemicals or radicals. Bymodulating the duty cycle of plasma production, the flow characteristicsof the neutral gas during the plasma off time (or reduced-power-leveltime) can be adjusted to control the uniformity of the process by way ofthe duty cycle. Since the plasma initiation time is usually within 10-20microseconds of the application of the RF, the duty cycle may becontrolled at frequencies as high as tens or hundreds of kHz.

In a preferred embodiment, a helicon mode RF wave is used for ignitingand generating the plasma. However, other modes in addition to theillustrative helicon mode may also be used. The plasma source can, forexample, operate as a type of inductively coupled plasma (ICP) device aswell. In addition, variations are suitable for capacitively coupled mode(E-mode) operations.

BRIEF DESCRIPTION OF THE FIGURES

The following illustrative figures are provided to better explain thevarious embodiments of the invention without intending for the figuresto limit the scope of the claims.

FIG. 1 illustrates a plasma source chamber with two sets of antennaelements;

FIG. 2 illustrates a tunable circuit with an RF power source coupled toan antenna;

FIG. 3 illustrates a second tunable circuit with an RF power sourcecoupled to an antenna;

FIG. 4 illustrates a third tunable circuit with an RF power sourcecoupled to an antenna;

FIG. 5 illustrates a circuit with an RF power amplifier coupled to anantenna current strap;

FIG. 6 illustrates a second circuit with an RF power amplifier coupledto an antenna current strap;

FIG. 7 illustrates a third circuit with an RF power amplifier coupled toan antenna current strap;

FIG. 8 illustrates a simplified model of the RF power amplifier, antennacurrent strap, and plasma;

FIG. 9 illustrates a lumped circuit equivalent of the model depicted inFIG. 8;

FIG. 10 illustrates the frequency response of a plasma source without aplasma;

FIG. 11 illustrates the frequency response of a plasma source with aplasma present; and

FIG. 12 illustrates a feedback arrangement for controlling a plasmasource.

DETAILED DESCRIPTION OF THE INVENTION

Turning first to the figures, FIG. 1 illustrates a plasma source chamberwith two sets of antenna elements configured in accordance with anembodiment of the present invention. The antenna design includes twoorthogonal single- or multi-turn loop elements 105, 110, 115, and 120,arranged about a common axis. The antenna elements 105, 110, 115, and120 are each driven by RF power sources, A 125 or B 130 as shown. Eachantenna loop may be coupled to the same RF power source with a phasesplitter, or to distinct RF power sources, to drive the antenna elementsin quadrature. Preferably the loops in the antenna are constructed fromeight (8) gauge teflon coated wire although copper wire or otherconductors may also be used.

FIG. 1 shows two orthogonal sets of two-element Helmholtz-coil-like loopantennas, with loop elements 105 and 115 in one set and loop elements110 and 120 in the second set. The loop elements are wrapped azimuthallyaround an insulating cylinder 135 such that the magnetic fields that areproduced when a current is passed through them are approximatelytransverse to the axis of the cylinder. The opposing elements of eachset are connected in series, in a Helmholtz configuration. The wiresinterconnecting opposing loop elements are preferably arranged such thatadjacent segments carry currents flowing in opposite directions in orderto enhance cancellation of stray fields associated with them, althoughthis is not necessary to the device operation. The antennas areenergized such that the currents in both orthogonal branches are nearlyequal and phased 90 degrees apart to produce an approximation to arotating transverse magnetic field.

In the example case of a helicon mode plasma, a static axial B_(o)-field140 is produced, for instance, by a simple electromagnet. This fieldruns along the axis of the cylinder. The direction of this static fieldis such that the rotating transverse field mimics that of the m=+1helicon wave. In practice, the amplitude and direction of the currentproducing the external field may be adjusted to modulate the performanceof the plasma generator. The overall amplitude of the necessary field istypically in the range 10-100 Gauss for the parameters discussed here,but for different size sources alternative ranges may be employed. Oncethe static field optimum amplitude and direction are chosen, theytypically need no further adjustment.

In combination, the static field and the RF field of the antennaelements produce the m=+1 helicon mode in the plasma inside theinsulating cylinder, which sustains the plasma discharge. It should benoted that it is also possible to vary, and thus de-tune the staticmagnetic field, or to not apply the field at all, so that the heliconmode is not directly excited. This operation produces a plasma as well,but typically not as efficiently as the helicon mode. Of course, thestatic field may then be applied to improve the operation of the plasmasource/generator.

It should also be noted that it is possible to achieve the same overallconditions of FIG. 1 using for instance multi-turn loop antennas insteadof single loop, and/or a squat bell jar. Although not a requirement, itis preferable for the Bell jar to fit within the antenna frame with nomore than a ½″ gap.

One example plasma source setup is as follows: A quartz bell jar hasapproximately 12″ inside diameter (such as a standard K. J. Lesker12×12), consisting of a straight-cylindrical section approximately 15 cmtall with a 6″ radius hemispherical top. The jar rests atop a vacuumchamber approximately 12″ i.d.×8″ tall (not part of the plasma source).The antennas consist of two sets of opposing, close-packed,approximately rectangular, two-turn continuous loop antenna elementsthat surround the bell jar, with approximately ⅛″ to ½″ spacing betweenthe antennas and the bell jar at every point. The turns within eachelement are connected in series, and the two elements within each setare also connected in series, such that their fields are additive. Theself-inductance of each set is approximately 10 microHenries in thisexample, and the mutual inductance between the two sets is less than 1microHenry. Vertical and horizontal antenna loop sections approximately25 cm and 20 cm long, respectively, consist of 8-guage Teflon-coatedwire. In alternative embodiments single turns of rigid copper conductorsmay be employed in place of one or two turns of Teflon-coated wire. Theparticular embodiments described herein for producing a transverserotating field are not intended to limit the scope of the invention.

A conventional RF power source and matching scheme, see FIGS. 2 to 4,may be used to excite the antenna currents in the antenna describedabove. Moreover, the circuits of FIGS. 2 to 4 are compatible with themethods of the present invention. These methods include steps such asproviding a low output impedance to an RF power source; and adjusting areactance coupling the RF power source to the antenna such that theresonance frequency in the absence of a plasma is the desired RFfrequency. A low output impedance can be understood by reference to thequality factor (“Q”) for the circuit with and without the plasma. The“Q” with no plasma present should be five to ten-fold or even higherthan in the presence of the plasma. Notably, unlike known circuits, sucha combination of the RF power source and antenna will not need to bereadjusted in the presence of plasma by changing the reactance inresponse to changes in the plasma impedance.

In FIG. 2 the RF source 200 may be a commercial 2 MHz, 0-1 kW generator,connected to the quadrature/hybrid circuit at port “A” 125 illustratedin FIG. 1 via 50 ohm coax. The “+45 degree” and “−45 degree” legs of thequadrature/hybrid circuit are connected to individual L-typecapacitative matching networks composed of adjustable capacitors 205,210, 215, and 220 as shown. The reactance of capacitors 225 is about 100ohms each at the operating frequency, and the reactance of either sideof the transformer 230 is about 100 ohms with the other side open. Asshown in FIG. 2, a single RF source 200 may be used, together with apassive power splitter (the quadrature/hybrid circuit) and fouradjustable tuning elements 205, 210, 215, and 220 to match to the twoseparate antenna inductances 235 and 240.

Another embodiment, illustrated in FIG. 3, employs two separate RF powersources 305 and 310, and thus entirely separates the two antenna powercircuits connected to inductances 335 and 340 via tunable capacitors315, 320, 325, and 330 respectively. Such an arrangement is advantageousin that each RF source can be operated at full power, thus doubling theamount of input power as compared to that of a single RF source, and thephasing and amplitude ratio may be adjusted between the antennas.Typically, sources 305 and 310 are operated at roughly the sameamplitude and at 90 degrees out of phase, although the amplitude and/orphase difference might be varied in order to change the nature of theexcited mode. For example, by operating them at different amplitudes, anelliptically polarized plasma helicon mode rather than a strictlycircularly polarized mode could be sustained.

A third embodiment, illustrated in FIG. 4, places a passive resonantcircuit, comprising inductor/antenna inductance 405 and adjustablecapacitor 410 on one leg, and drives the other leg with an RF source 400with a matching circuit having tunable capacitors 415 and 420 connectedto antenna inductance 425. This arrangement tends to excite the samesort of elliptical helicon mode in the plasma, with the passive sideoperating approximately 90 degrees out of phase with the driven side,thus providing many of the advantages of the invention but with only asingle RF source and matching network.

The working gas in this example setup is Argon, with pressure rangingfrom 10 mTorr to over 100 mTorr. A static axial field is manuallysettable to 0-150 G and is produced by a coil situated outside the belljar/antenna assembly, with a radius of about 9″.

Plasma operation at a pressure of approximately 75 mTorr exhibits atleast three distinct modes. First, a bright mode in which the plasma isconcentrated near the edge of the bell jar is observed forB_(o)<B_(critical) when P_(RF) is less than or approximately 200 W.Here, B_(o) is the axial magnetic field while B_(critical) is a criticalvalue for the axial field for exciting a plasma using a helicon mode.Similarly, power levels P_(RF) and P_(threshold) denote the RF powersupplied to the antenna and a threshold power described below. In thismode, the RF antenna currents tend to not be in quadrature, insteadbeing as much as 180 degrees out of phase. Second, adull-glow-discharge-like mode, with uniform density/glow at higher powerbut with approximately 1-2 cm thick dark space along the wall of thebell jar at lower powers, is observed for B_(o)>B_(critical) butP_(RF)<P_(threshold). In this case the RF currents are in robustquadrature, appearing to abruptly lock at approximately 90 degrees phaseshift shortly after the plasma is formed. Third, at higherP_(RF)>P_(threshold) and with B_(o)>B_(critical), a bright plasma isformed that appears to be more evenly radially distributed than that ofmode (1), and the antenna currents again tend to lock into quadraturephasing. The third regime represents an efficient mode of operation, andcan be achieved at a neutral gas pressure that has proven to be verydifficult to access for known plasma sources, although each of theseregimes may have application in plasma processing.

In an aspect, the present invention also enables the elimination of theconventional RF power source and tunable matching network described inFIGS. 2 to 4, in favor of a streamlined power circuit.

In a preferred embodiment of the present invention, an RF power circuitdrives the antenna current strap directly, using an arrangement such asthat shown in FIG. 5. The RF amplifier illustrated in FIG. 5 ispreferably one of the many types of RF amplifiers having a low outputimpedance (i.e. a push-pull output stage) that are known in the field.Transistors 505 and 510 are driven in a push-pull arrangement byappropriate circuitry 500, as is known to one of ordinary skill in theart. In this arrangement only one or the other transistor is conductingat any time, typically with a duty cycle of or less than 50%. The outputof the two transistors is combined to generate the complete signal.

In a preferred embodiment, the power semiconductors, e.g., transistors505 and 510, in the output stage are operated in switching mode. In theFIGS. 5-7 these are depicted as FETs, but they can also be, for example,bipolar transistors, IGBTs, vacuum tubes, or any other suitableamplifying device. An example of switching mode is provided by Class Doperation. In this mode alternate output devices are rapidly switched onand off on opposite half-cycles of the RF waveform. Ideally since theoutput devices are either completely ON with zero voltage drop, orcompletely OFF with no current flow there should be no powerdissipation. Consequently class D operation is ideally capable of 100%efficiency. However, this estimate assumes zero on-impedance switcheswith infinitely fast switching times. Actual implementations typicallyexhibit efficiencies approaching 90%.

The RF driver is then coupled directly to the antenna current strap 520through a fixed or variable reactance 515, preferably a capacitor. Thiscoupling reactance value is preferably such that the resonant frequencyof the circuit with the coupling reactance and the antenna, with noplasma present, is approximately equal to the RF operating frequency.

An alternative arrangement of the output stage of this circuit,illustrated in FIG. 6(A), includes a transformer 620 following orincorporated into the push-pull stage, with driver 600 and transistors605 and 610, to provide electrical isolation. Transformer 620 mayoptionally be configured to transform the output impedance of thepush-pull stage, if too high, to a low impedance. Capacitor 615 isarranged to be in resonance at the desired drive frequency with theinductive circuit formed by transformer 620 and antenna current strap625. A similar embodiment is shown in FIG. 6(B), where capacitor 630 isused for DC elimination, and capacitor 635 is resonant in the seriescircuit formed by leakage inductance of transformer 620 and inductanceof the current strap 625.

FIG. 7 illustrates yet another RF power and antenna current strapconfiguration in accordance with the present invention. A center-tappedinductor 725 incorporated in the DC power feed is connected to theoutput stage having push-pull driver 700 and transistors 705 and 710.Isolation is provided by transformer 720. Again, only one or the othertransistor is conducting at any time, typically with a duty cycle ofless than 50%. The circuits of FIGS. 5-7 are provided as illustrativeexamples only. Any well-known push-pull stage or other configurationsproviding a low output impedance may be used in their place.

The RF power source may also be used with any helicon antenna, such aseither a symmetric (Nagoya Type III or variation thereof, e.g.,Boswell-type paddle-shaped antenna) or asymmetric (e.g., right-handhelical, twisted-Nagoya-III antenna) antenna configuration, or any othernon-helicon inductively coupled configuration.

The RF power source may be amplitude modulated with a variable dutycycle to provide times of reduced or zero plasma density interspersedwith times of higher plasma density. This modulation of the plasmadensity can be used to affect the flow dynamics and uniformity of theworking gas, and consequently the uniformity of the process. A morespatially uniform distribution comprising plasma may therefore begenerated by a plasma generator system in accordance with the presentinvention by a suitable choice of a modulation scheme.

In general, a plasma generator system in accordance with the presentinvention may use radio frequency power sources based on operation as asubstantially Class A amplifier, a substantially Class AB amplifier, asubstantially Class B amplifier, a substantially Class C amplifier, asubstantially Class D amplifier, a substantially Class E amplifier, or asubstantially Class F amplifier or any sub-combination thereof. Suchpower sources in combination with the antennas for exciting helicon modeare suitable for generating high density plasmas. Moreover, fornon-switching amplifiers, such as those shown in FIGS. 2-4, anintermediate stage to transform the RF source impedance to a low outputimpedance may be employed to approximate the efficient operation of theswitching amplifier based embodiments described herein.

In inductively coupled plasma sources, the antenna current strap islocated in proximity to the region where plasma is formed, usuallyoutside of an insulating vessel. From a circuit point of view, theantenna element forms the primary of a non-ideal transformer, with theplasma being the secondary. An equivalent circuit is shown in FIG. 8, inwhich inductor 810 represents a lumped-element representation of thecurrent strap and any inductance in the wiring, including any inductanceadded by e.g., the driver's output transformer present in someembodiments. Components in the box labeled P represent the plasma:inductor 820 is the plasma self inductance, and impedance 815 representsthe plasma dissipation, modeled as an effective resistance. M representsthe mutual inductance between the antenna and plasma. Transistor driver800 is represented as a square-wave voltage source. The capacitance 805is adjusted at the time the system is installed to make the resonantfrequency of the circuit approximately match the desired operatingfrequency. In an alternate embodiment with a fixed capacitor, the RFfrequency may be adjusted to achieve the same effect.

For illustrating the operation of the system, the overall system may bemodeled as shown in FIG. 9. In FIG. 9 all inductors have been lumpedinto inductance 905, all capacitors into capacitance 910, and alldissipating elements into resistor 915, and the amplifier should ideallyoperate as an RF voltage source (i.e., having zero output impedance).

With no plasma present, R is small since there is little dissipation,and the circuit of FIG. 9 exhibits a narrow resonant response to changesin frequency, as shown in FIG. 10. This provides one of the advantagesof the circuit's operation: it is possible to drive the voltage on theantenna to a high value with relatively little power input, thusfacilitating the initial breakdown of the gas in the reaction chamber.Once the plasma forms, the damping in the system considerably broadensthe resonant peak, as shown in FIG. 11, reducing the Q of the overallcircuit. Although the center frequency of the resonance may shift withplasma conditions, that shift is negligible compared to the width of theresonant response when the plasma load is present. Therefore, whenoperating with a plasma load the circuit is relatively insensitive tovariations in operating conditions, and requires no retuning. This isillustrated in FIG. 11, where the overall system resonance has shiftedits frequency slightly, although the Q is sufficiently reduced that theoperation of the system remains efficient. With the reduced Q of thecircuit, the voltage applied to the plasma self-adjusts to beconsiderably reduced over the no-plasma case. In some embodiments, itmay be somewhat advantageous to actually detune the operating frequencyof the RF drive slightly from the exact no-plasma resonance to one sideor the other, depending on the shift of the resonant frequency when theplasma forms.

The level of power input to the plasma may be controlled by a variety oftechniques, such as adjusting the DC supply level on the RF outputstage. In one embodiment, the supply voltage may be in response tosensed variations in plasma loading to maintain a relatively constantpower into the plasma source. As illustrated in FIG. 12, the sensing ofplasma loading for adjustments by DC supply regulator 1230 may beachieved, for example, by monitoring the voltage from the DC supply 1215by voltage sensor 1200 and the DC current into the RF/Plasma system bycurrent sensor 1205, and using their product together with a previouslymeasured approximation to the amplifier efficiency in module 1210 toestimate the net power into the plasma 1225 from RF Amplifier 1220.Efficiency multiplier for gain module 1235 can be measured for differentoutput levels, for instance by monitoring heat loads at various pointsof the system, and stored digitally, so that variations in efficiencywith output level are accounted for. Alternatively, the RF voltage andcurrent can be measured, and their in-phase product evaluated toestimate the real power being dissipated in the plasma.

The sensing of plasma may also extend to sensing spatial uniformity byeither direct sensing or indirect sensing by way of variations in thevoltage or current. Changing the duty cycle in response to suchvariations can then control the spatial distribution of plasma. Inaddition, modulating the duty cycle can further allow control over theaverage input power to improve the efficiency of plasma generation. Thefeedback arrangement of FIG. 12 can also allow switching between two ormore power levels as described previously.

“Low” impedance, as used herein, means that the series resonant circuitshown in FIG. 9 has a “Q” that should be five to ten-fold or even higherwith no plasma present than with plasma present. That is, the amplifieroutput impedance should be sufficiently small that the energy dissipatedin a half-cycle of output is much less than that stored in the reactivecomponents. This condition is mathematically defined asZ_out<<sqrt(L/C), where L and C are the lumped values shown in FIG. 9.The RF amplifier will approach operation as a voltage source when thiscondition holds.

Prior to plasma initiation the reaction chamber is filled with a workinggas particular to a given process. This invention provides an advantagein being able to break down this gas and initiate the plasma by virtueof the fact that the high Q of the circuit with no plasma allows highvoltages to be induced on the antenna element with relatively littlepower in the absence of a plasma. This no-plasma voltage can becontrolled to give a programmed breakdown of the working gas; once theplasma forms, induced currents in the plasma serve to load the system sothat these higher voltages decay and thus, avoid stressing the system.

The described circuit arrangements, in accordance with the presentinvention, do not require a variable tuning element, such as amechanically adjustable capacitor, since only fixed capacitance C isnecessary. However, the various circuits can also be constructed using avariable capacitor that is adjusted, for example, for matching of thesystem resonance to the desired operating frequency, in a preferredembodiment, and is not needed for real-time impedance matching with theplasma operating point. Such matching is useful to counter the effectsof mechanical vibration or aging that may cause the L-C resonantfrequency to drift.

In one embodiment, the operating frequency is adjusted to compensate forsmall deviations from resonance, while mechanically tuning the capacitorcompensates for large deviations. In an alternative embodiment,adjustments are made by tuning the capacitor. In the preferred (tuned)embodiment, this tuning is automated and takes place during periods whenthe source is offline. In another aspect, with tuning as part of theprocess control, for instance to provide small tweaks to the processconditions, the disclosed arrangement reduces the number of adjustableelements to as few as one in embodiments with adjustable tuningelements.

As one skilled in the art will appreciate, the disclosed invention issusceptible to many variations and alternative implementations withoutdeparting from its teachings or spirit. Such modifications are intendedto be within the scope of the claims appended below. For instance, onemay provide impedance matching for a low impedance with a transformer incombination with a conventional amplifier. Therefore, the claims must beread to cover such modifications and variations and their equivalents.Moreover, all references cited herein are incorporated by reference intheir entirety for their disclosure and teachings.

1. A method of making a plasma source to eliminate the need for amatching circuit, the method comprising the steps of: providing a lowoutput impedance to a radio frequency power source; selecting acapacitance coupling the radio frequency power source to at least oneset of antenna loops such that the capacitance and the antenna loopswithout a plasma have a resonant frequency that is about equal to aspecified frequency for the plasma; and controlling an average inputpower by modulating a duty cycle for operating the radio frequency powersource.
 2. The method of claim 1 further comprising the step of sensinga spatial distribution of the plasma, and in response thereto modulatingthe duty cycle to provide times of neutral gas flow, thereby modulatingthe spatial distribution of the plasma.
 3. The method of claim 1 furthercomprising the step of sensing a spatial distribution of the plasma, andin response thereto modulating the duty cycle to provide times ofneutral gas flow, thereby modulating the spatial distribution of aworking gas.
 4. The method of claim 1 further comprising the step ofmodulating the duty cycle to provide for neutral gas flow.
 5. The methodof claim 4, wherein the duty cycle is modulated at a frequency selectedfrom at least about 1 Hz, at least about 10 Hz, at least about 100 Hz,at least about 500 Hz, at least about 1000 Hz, at least about 5000 Hz,at least about 10,000 Hz and at least about 100,000 Hz.
 6. The method ofclaim 1, wherein a plasma power is switched between two or more levels.7. The method of claim 6, wherein a plasma power is switched from about30 percent to about 100 percent of full power.
 8. The method of claim 1,wherein the average input power is selected from about 5 watts, about 10watts, about 5 to 10 watts, and about 10 to 50 watts.
 9. The method ofclaim 8, wherein the average input power is applied at an averagedensity of about 1 watt per 10 liters of volume.
 10. A method ofoperating a plasma source to eliminate the need for a matching circuit,the method comprising the steps of: providing a low output impedance toa radio frequency power source; selecting a capacitance coupling theradio frequency power source to at least one set of antenna loops suchthat the capacitance and the antenna loops without a plasma have aresonant frequency that is about equal to a specified frequency for theplasma; and modulating a spatial distribution of the plasma by changinga duty cycle for operating the radio frequency power source.
 11. Themethod of claim 10, wherein the duty cycle is modulated at a frequencyselected from at least about 500 Hz, at least about 1000 Hz, at leastabout 5000 Hz, and at least about 10,000 Hz.
 12. The method of claim 10further comprising the step of sensing a spatial distribution of theplasma, and in response thereto modulating the duty cycle.
 13. Themethod of claim 10, wherein the duty cycle is selected to be in a rangedefined by two of about 10 percent, about 30 percent, about 50 percent,about 80 percent, about 90 percent, and about 100 percent.
 14. Themethod of claim 10 further comprising the step of controlling theaverage input power by modulating the duty cycle.
 15. The method ofclaim 14, wherein the average input power is selected from about 5watts, about 10 watts, about 5 to 10 watts, and about 10 to 50 watts.16. The method of claim 15, wherein a plasma power is switched in arelatively short time between two or more levels.
 17. The method ofclaim 16, wherein a plasma power is switched from about 30 percent toabout 100 percent of full power.